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ADS7957: Datasheet is unclear about VQFN layout, doesn't define pad

Azad Jalali
Prodigy 40 points
Part Number: ADS7957

I'm trying to solve a problem that has an obvious answer but no explicit information in the datasheet. The VQFN package for this part presumably has an underside pad which is presumably connected to ground, but the datasheet doesn't acknowledge this pad at all. Although it's unfathomable that the pad be connected to anything but ground in an ADC, I'd still expect to see that mentioned somewhere in the datasheet. In fact, the entire Layout section is written as if the VQFN option doesn't exist. I have two questions:

  1. Can someone from TI confirm that the pad is ground, just so I have something official to point to during a design review? Or, if I missed something obvious in the datasheet/general knowledge, can someone more attentive than me let me know?
  2. Do the layout guidelines in Section 12 apply in principle to the VQFN package as well? Specifically calling for >=4 vias between the copper fill (which in this case is a pad) and the ground plane?

Thank you,

Azad

  • 0
    TI__Expert 3175 points
    Hi Azad,

    Great questions, and your intuition is correct.

    1) The pad on the ADS7957 VQFN package should be connected to the analog ground on your board. This is a low power device (14.5 mW typical) so heat dissipation is part of the reason but not a big concern. Instead this pad primarily serves as a low impedance, low inductance path to ground as well as provide a stronger mechanical connection between the device and board.

    2) The layout guidelines in section 12 do apply to the VQFN package. Regarding the vias specifically, at least 4 should be used. An example layout using 9 vias (which provides a better path to ground than 4 vias) can be seen on page 64 of the datasheet.
  • 0 in reply to Evan Sawyer
    Prodigy 40 points

    Hey Evan,

    Thanks for getting back to me. I did see page 64 but because it talked about thermals I figured it was a generic drawing and not necessarily applicable. Anyway I appreciate the clarification!

    Best,

    Azad 

  • 0 in reply to Azad Jalali
    TI__Expert 3175 points

    Hi Azad,

    Happy to help and please let us know if there is anything else we can do to assist!